We would like to attach a copper wire to a piece of ductile iron. What is the best method? Is it possible to solder the wire? If so, what solder should be used?
I would braze the copper wire to ductile iron. The web site below offers some recommended braze metals for dissimilar metal applications. The key is to have a locally clean surface and use of a suitable flux;
You certainly can try silver solder/flux. I have had mixed results with silver/tin solders on cast irons. I prefer brazing because the higher heat input drives out contaminants.
We add very small wires to CI/DI parts by making a small brazed area, preform furnace, then soldering the wire to it in the brazed area. This enable us to replace the wires without removing the parts from the machine.
Prior to this there were several Cd base solders that worked very well on CI/DI.